Improve concurrency access for updateMetrics()

nuplayer's updateMetrics() referenced an unprotected shared stats buffer.
It's a small buffer, so we now make a copy during updateMetrics()
[at a point where we are mutexed] instead of putting a mutex on the
underlying frequently used construct.

Ensure that nuplayer2 has the same protections.

Bug: 123256408
Test: race condition
diff --git a/media/libmediaplayer2/nuplayer2/NuPlayer2Decoder.cpp b/media/libmediaplayer2/nuplayer2/NuPlayer2Decoder.cpp
index 9729d86..66bfae5 100644
--- a/media/libmediaplayer2/nuplayer2/NuPlayer2Decoder.cpp
+++ b/media/libmediaplayer2/nuplayer2/NuPlayer2Decoder.cpp
@@ -109,7 +109,10 @@
     mStats->setInt64("frames-dropped-output", mNumOutputFramesDropped);
     mStats->setFloat("frame-rate-total", mFrameRateTotal);
 
-    return mStats;
+    // i'm mutexed right now.
+    // make our own copy, so we aren't victim to any later changes.
+    sp<AMessage> copiedStats = mStats->dup();
+    return copiedStats;
 }
 
 status_t NuPlayer2::Decoder::setVideoSurface(const sp<ANativeWindowWrapper> &nww) {